The present invention relates generally to the field of forming three-dimensional objects. Specifically it relates to volumetric feed control of a build material element in three-dimensional modeling.
Three-dimensional modeling machines in the current art often use build elements made from a build material such as a filament, often wound on a roll, that are fed to the machine by the use of feed rollers or other devices. In the modeling machine, the material is melted before being dispensed at an application tip. The filament is preferably made of a material that will rapidly solidify upon a small drop in its temperature. Materials often used for modeling include waxes, thermoplastic resins, and various metals.
Build material elements have varying configurations, but typical elements used in the field of three-dimensional modeling must currently be manufactured to a high degree of accuracy due to the tolerance limits imposed by current modeling machine technology. The tolerance limits imposed on build material elements by the current technology are in the area of 4 per cent of volume. Typically, the build material element is in the form of a cylindrical filament approximately 0.070 inches in diameter, with a required tolerance of +/-0.0015 inches. In current modeling systems using this filament, if the filament exceeds tolerance by either being too large or too small, the machine may plug or the part become corrupted. Also in current systems, the accuracy of the model created is dependent upon the accuracy of the filament supply. When the filament varies in cross section, the volume of material delivered to the application tip of the machine does not remain constant, and therefore the flow rate of material fluctuates. When the filament is too large, there may be a build-up of excess filament at the application tip, resulting in a model layer that is too thick. Similarly, when the filament is too small, there may be a shortfall or lag of filament at the application tip, resulting in a deposited layer that is too thin, or that leaves gaps in the model. Both situations may easily lead to model failure. An uncontrolled flow rate, even when the fluctuations are small, can contribute to large variations in the quality of the final model, including gaps and excess thickness.
The three-dimensional modeling machines of the current art use flexible filament, often stored on a roll, that is fed to an application tip by the use of feed rollers. Although attempts have been made at constant volumetric control of melted filament at the application tip, proposed solutions have not controlled the problem enough to allow a tolerance level change in the filament used. An excess of material leads to overthick layers, and a shortfall leads to unacceptable gap or thin wall errors in modeling.
Various prior art designs for automatic feed control and variable speed feeding exist. For example, U.S. Pat. No. 3,017,060 to Miller discloses an automatic feed control mechanism for wire filaments. Miller teaches an intermittent feed of the filament, the filament having segments of varying cross-sectional area. No continuous measuring of the wire diameter, cross sectional area, or effective cross section is taught by Miller. No continuous adjustment of filament feed speed in response to changes in wire diameter is taught by Miller.
U.S. Pat. No. 4,932,581 to Ohle et al. discloses the controlled speed feeding of a predetermined length of solder wire to a soldering location. Ohle et al. uses force sensors to determine feed force or speed by converting force to a proportional electric signal controlling an electric motor. Ohle et al. does not teach any type of sensing device for measuring the diameter, cross sectional area, or effective cross section of the solder wire. Adjustment of the feed controller for solder wire thickness is done manually, and only for a nominal diameter. No adjustments are made due to any variance from nominal diameter.
Several patents disclose various devices to control feeding of fibers or yarn. For example, U.S. Pat. No. 4,019,691 to Wroclawski et al. discloses winding of yarn onto a bobbin at a variable and controlled speed. The speed is dependent upon the volume of yarn held in a receptacle. Sensors in the receptacle are of variable type, including photoelectric sensors. The sensors determine the volume of yarn in the receptacle. The winding shaft in Wroclawski et al. rotates at a constant speed. The variable yarn winding speed is accomplished by use of a coneshaped bobbin. A different contact point on the bobbin results in a different yarn winding speed. Wroclawski et al. discloses no monitoring or sensing of the diameter of the yarn to control feed speed. The feed speed of the yarn is constant.
U.S. Pat. No. 4,820,281 to Lawler, Jr. discloses measurement of the volume of a drop of liquid and control means for regulating flow of a liquid. The volume of drops of liquid in an IV tube is calculated by timing circuits activated by photoelectric sensors to measure the time it takes for the drop to fall a known distance. Volumetric flow rate is determined by multiplying drop volume and drop rate, and is adjusted according to a predetermined flow rate. No continuous feeding of a solid element is taught by Lawler, Jr. No continuous monitoring of diameter, cross sectional area, or effective cross section of a filament is taught by lawler, Jr. The teaching of Lawler, Jr. is unworkable when dealing with solids.
It would be desirable to provide an apparatus for continuous volumetric control of build material fed to the extrusion head of a three-dimensional modeling machine. Such an apparatus should be adaptable to be added to an existing machine, as well as added at the manufacturing stage.